1. Field of the Invention
The present invention relates to novel bridged clays having a large interfoliate spacing and to a process for the preparation of such novel bridged clays.
2. Description of the Prior Art
It is known to this art that certain clays have a structure characterized by an expandable lattice. In effect, they are capable of absorbing water, in particular between the different lamella thereof. Clays of the smectite and vermiculite categories exhibit this property. The structure of these clays may be simply described as triple layer lamella having an approximate thickness of 1 nm, containing two layers of SiO.sub.4 tetrahedrons, separated by a layer of MX.sub.6 octahedrons, in which M is a trivalent ion, for example Al.sup.3+ in the case of dioctahedron clays or a divalent ion, for example Mg.sup.2+ in the case of trioctahedron clays: X is an oxygen atom, a hydroxyl group OH or a fluorine atom F.
The silicon atoms of the tetrahedrons may be replaced in part by aluminum atoms and the aluminum atoms and magnesium atoms of the octahedrons may be respectively substituted, in particular, by magnesium or iron atoms, and lithium atoms.
Because of the expandable nature of such clays, they are used in the field of catalysis, where the largest possible surface area accessible to the reagents is desired.
However, the application of nonbridged clays as catalysts or catalyst supports is limited to catalysis in the liquid phase, as expanded clays undergo desiccation beginning at the boiling temperature of the solvent. This results in a loss of their expanded nature and, therefore, of their accessible interlamellar surface area.
This art has sought to expand clays by a different technique, to permit their employment as catalysts or catalyst supports in the gaseous phase also.
Thus, it is known to expand the clays by introducing or inserting a mineral species between the clay lamella, thereby creating pillars or bridges. Hence, the clays produced in this manner are designated "bridged clays".
A well-known technique entails establishing the bridging by means of oligomers of a metal hydroxide, in particular aluminum.
Typically, such bridging of clays is carried out via a three-stage process:
(i) preparation of the aluminum pillar Al.sub.13 of the formula: [Al.sub.13 O.sub.4 (OH).sub.24+x (H.sub.2 O).sub.n ](7-x)+ (with x ranging from 0 to 4 and n characteristically ranging from 8 to 12) from an aluminum salt and a base;
(ii) mixing the solution containing the source of Al.sub.13 with the aqueous clay solution; and
(iii) washing or dialysis of the final product.
In particular, FR 2,512,043 describes a process of the above type.
The process described in FR 2,512,043 permits synthesis of a bridged aluminum clay, the basal spacing of which is a maximum of 1.95 nm, corresponding to an interfoliate spacing of at most 1 nm.
As utilized herein, by the expression "interfoliate spacing" is intended the spacing between two lamella and by the expression "basal spacing", represented by d.sub.001, is intended the sum of the thickness of a lamella and the interfoliate spacing.
Microporous materials having a bridged lamellar structure may present desirable form selective properties in catalysis, if the size of their pores (porosity) corresponds to that of the molecules of the reagents and the product of the reaction. But the molecules used in fine chemistry frequently are larger than 1 nm, which renders bridged clays having a smaller interfoliate spacing useless for such applications.